Hello John,
You can set this up securely just by allowing access from specific users so no one else can access that particular Access Exchange Server on the inside of your network.
You can post the configuration an we can let you know if it can be more secure,
Regards,
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC